Swine Variant Flu Season

CREDIT CDC

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For the past dozen or so years, mostly during the summer and fall, we have been on watch for reports of novel swine variant flu infections in humans, which are usually associated with swine exposure at county or state fair exhibits, or the raising of pigs. 

The CDC describes Swine Variant viruses in their Key Facts FAQ.

What is a variant influenza virus?

When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant influenza virus.” For example, if a swine origin influenza A H3N2 virus is detected in a person, that virus will be called an “H3N2 variant” virus or “H3N2v” virus.

While there are hundreds of county and state fairs every summer and fall across the country who receive millions of visitors, we generally only hear about a handful reporting influenza in pigs, with fewer still resulting in confirmed human infections.

In the fall of 2017, we looked at an EID Journal Dispatch (Transmission Of Swine H3N2 To Humans At Agricultural Exhibits - Michigan & Ohio 2016), that found while widespread illness in pigs was only rarely reported, surveillance revealed an average prevalence of influenza A in fair pigs of  77.5%.  

This study cautioned that this suggests `. .  . that subclinical influenza A infections in pigs remain a threat to public health (3).'

In other words, healthy looking pigs can carry, and transmit swine-variant viruses.  We saw similar findings in a 2012 study (see EID Journal: Flu In Healthy-Looking Pigs).

In 2012 more than 320 human infections were reported around the nation, while 5 years ago (2016/17) 66 cases were recorded. In 2013 we looked at a study (see CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012) that suggested the number of cases could be as much as 200 times higher than reported.

Over the last year, only 4 cases were reported in the United States, although other countries (including Canada, France and Denmark) reported cases as well.  Surveillance, and testing for these viruses is far from robust, however, and so many infections likely go undetected. 

Since the influenza subtypes that commonly circulate in swine (H1, H2 & H3) are also the same HA subtypes as have caused all of the human pandemics going back 130 years (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?), when swine variant viruses jump to humans, it gets our attention.

Pigs are viewed as excellent `mixing vessels' for  influenza viruses, due to having both mammalian α2,6 receptor cells and avian-like α2,3 receptor cells, and having frequent contact with humans and birds. They are often infected with human seasonal flu, along with their own swine flu viruses.

Although most swine variant infections have been mild or moderate, a couple of deaths have been reported since 2012, along with a number of hospitalizations. The CDC's IRAT (Influenza Risk Assessment Tool) lists 3 North American swine viruses as having at least some pandemic potential (2 added in 2019). 

H1N2 variant [A/California/62/2018]  Jul   2019   5.8  5.7 Moderate

H3N2 variant [A/Ohio/13/2017]          Jul   2019   6.6  5.8 Moderate

H3N2 variant [A/Indiana/08/2011]      Dec 2012   6.0  4.5 Moderate 

In 2021 the CDC ranked  a Chinese Swine-variant EA H1N1 `G4' as having the highest pandemic potential of any flu virus on their list (see EID Journal: Zoonotic Threat of G4 Genotype Eurasian Avian-Like Swine Influenza A(H1N1) Viruses, China, 2020).

In last Friday's MMWR report (Influenza Activity and Composition of the 2022–23 Influenza Vaccine — United States, 2021–22 Season), swine variant influenza - along with avian H5N1 - was mentioned prominently.  

Because of the atypical timing and duration of influenza activity, providers and patients should consider influenza infection as a cause of respiratory illness. Testing for seasonal influenza and monitoring for novel viruses, especially avian A(H5N1) and swine viruses, should continue year-round.

         (Snip) 

Sporadic seasonal influenza virus infections and novel influenza A virus infections associated with exposure to swine during animal exhibitions are often reported during summer months (6).  

After two-plus years of pandemic precautions (social distancing, mask wearing, etc.), community immunity to influenza viruses - which includes novel swine and avian viruses - is believed to be very low.  At the same time, influenza testing has declined in many parts of the world.  

Some studies have suggested that when community immunity is low, novel flu viruses have a better chance of sparking a pandemic (see PLoS Comp. Bio.: Spring & Early Summer Most Likely Time For A Pandemic).

While the risks from swine variant viruses is currently low, the H1N1 pandemic 2009 showed how quickly the status quo can change. 

The CDC maintains a comprehensive Swine/Variant Influenza page where you'll find the latest case counts, safety and prevention information, and guidance for health care providers.

Some information for exhibitors at county and state fairs include:

• Key Facts for People Exhibiting Pigs at Fairs[545 KB, 2 pages]
• Take Action to Prevent the Spread of Flu Between People and Pigs[1.3 MB, 2 pages]
• Educational Posters[389 KB, 1 page]
• Compendium of Measures to Prevent Disease Associated with Animals in Public Settings, 2013.
• Reduce Your Risk (English)[22 KB, 1 page] | (Spanish)[22 KB, 1 page]